This invention relates to rolling machines, as for the cold-rolling of metal, such as aluminum, to produce an elongate sheet of predetermined thickness, which may be a foil thickness.
In such machines, squeeze forces are in the order of hundreds of tons applied at a working pass via working rolls under compressional loading of back-up rolls, the ends of which are journaled for rotation in individual chocks. Massive frame or housing structure contains and mounts the chocks and their rolls, and the frame structure also fully contains all compressional forces delivered to the back-up rolls via their chocks. However, despite its massive nature, the frame yields elastically to the large forces involved, thus placing limitations on the fidelity with which thickness tolerances can be maintained on rolled product, particularly near the beginning or near the end of a given product run; moreover, elastic deformations of the frame impair the ability of chock-loading systems to respond to such transient changes in load as may be called for by sensed thickness, hardness, or width variations in input material, or by roll eccentricity, in the course of a given run. The chock-loading systems generally involve either motor-driven lead screws or hydraulic actuators, each of which is inherently a limiting factor on ability to respond quickly to transient load requirements.
In recognition of problems attributable to elastic deformation of the frame, it has been a practice to prestress the mill by electromagnetically setting the working-roll gap through a wedge assembly installed between roll chocks. The mill frame is placed under a constant pre-stress force which substantially exceeds the maximum rolling force, and this pre-stress force counteracts the rolling force to eliminate further housing stretch or deflection. The wedges are in paired opposition, and differentially actuated by motor-driven lead screws.
In another approach to the problem, U.S. Pat. No. 4,102,171 discloses load-transfer blocks between opposed chocks at each end of the mill, to relieve a part of the pre-stress force for each particular strip-rolling operation. At each load-transfer block, a combination of hydraulic pressure and gas pressure provides a controlled substantially unyielding force during a rolling operation, and a yielding shock absorber for preventing full prestress load from coming on the rolls when an end of the strip passes the rolls or when a strip breaks.
In the commercial manufacture of aluminum foil, from input aluminum sheet material, involving 50 percent thickness reduction at each rolling stage, it is customary to manufacture to an ultimate product-thickness tolerance of 5 percent. However, existing prestressing techniques do not assure that this tolerance requirement will necessarily be met, even though the system be finely adjusted for a 2.5 percent thickness tolerance, so as to have a 2:1 safety factor in respect of the specified 5 percent tolerance. The time constants of prestress control are not equal to the task of responding to roll eccentricity, varying thickness and hardness of input sheet material, at the increasing rate of rolling speed which competition compels, and to the more severe thickness tolerances which economics dictate.